Typical complementary metal-oxide-semiconductor (CMOS) image sensors sense light by converting impinging photons into electrons that are integrated (collected) in sensor pixels. Once the integration cycle is complete, collected charge is converted into a voltage signal, which is supplied to output terminals of an image sensor. This charge to voltage conversion is performed within each sensor pixel. The pixel output voltage (i.e., an analog voltage signal) is transferred to the output terminals using various pixel addressing and scanning schemes. The analog voltage signal can also be converted on-chip to a digital equivalent before reaching the chip output.
The sensor pixels include buffer amplifiers (i.e., source followers) that drive sensing lines connected to the pixels through address transistors. After the charge to voltage conversion and after the resulting voltage signal has been read out from the pixels, the pixels are reset in preparation for a successive charge accumulation cycle. In pixels that include floating diffusions (FD) serving as charge detection nodes, the reset operation is performed by turning on a reset transistor that connects the floating diffusion node to a voltage reference.
Removing charge from the floating diffusion node using the reset transistor, however, generates kTC-reset noise as is well known in the art. The kTC noise must be removed using correlated double sampling (CDS) signal processing technique in order to achieve desired low noise performance. Typical CMOS image sensors that utilize CDS require at least four transistors (4T) per pixel. An example of the 4T pixel circuit with a pinned photo-diode can be found in Lee (U.S. Pat. No. 5,625,210), incorporated herein as a reference.
In modern CMOS image sensor designs, circuitry associated with multiple photo-diodes may be shared, as can be found for example in Guidash (U.S. Pat. No. 6,657,665). In Guidash, a sensor pixel consists of two photo-diodes located in neighboring rows. The two photo-diodes located in the neighboring rows share the same circuitry. Sharing circuitry in this way can result in having only two metal bus lines in the row direction and two metal bus lines in the column direction per photo-diode, as shown in FIG. 1.
This is useful for designing small pixels or pixels with high fill factor (FF), because the minimum pixel size is dependent on the spacing and width of the metal bus lines. This is also illustrated in FIG. 1, where drawing 100 represents the schematic diagram of a shared circuit pixel with two photo-diodes 107 and 108. Photo-diodes 107 and 108 are coupled to common floating diffusion charge detection node 115 through charge transfer transistors 109 and 110. FD node 115 is connected to the gate of source follower (SF) transistor 112. SF transistor 112 has a drain that is connected to Vdd column bus line 101 (i.e., a positive power supply line on which positive power supply voltage Vdd is provided) via line 116. SF transistor 112 has a source that is connected to output signal column bus line 102 via address (Sx) transistor 113 and line 117.
FD node 115 is reset using transistor 111. Reset transistor 111 has a drain that is connected to line 116 and a source that is connected to node 115. Address transistor 113, reset transistor 111, and charge transfer transistors 109 and 110 are controlled using control signals supplied over row bus lines 114, 106, 104, and 105, respectively.
As shown in FIG. 1, the circuit that has two photo-diodes. This particular image sensor therefore has two row bus lines and two column bus lines per photo-diode. In many cases, however, it is also necessary to provide an additional connection between transistor 110 and FD node 115, as indicated by wire 103. This additional connection reduces the pixel fill factor.
Because reset transistor 111 is connected to supply line 101, the photo current drained from FD node 115 is mixed with the drain current flowing through SF transistor 112 and thus cannot be detected. This represents a disadvantage because the photo current corresponds to a sensor illumination intensity, which is often used to adjust the pixel charge integration time (for preventing pixel charge overflow). In standard configurations that lack direct photo current detection, the sensor array has to be read out several times, and a correct integration time is determined using a suitable search algorithm. This procedure consumes valuable time that may not be available in applications such as the automotive or endoscopic imaging.
It would therefore be desirable to be able to provide image sensors with photo current sensing capabilities.